<!DOCTYPE html>
<html>
    <head>
        <meta charset="UTF-8">
        <title>Star</title>
    </head>
    <body>
        <canvas id="universe"></canvas>
    </body>
    <style>
        body {
            background-color: #000000;
        }

        #universe {
            display: block;
            position: fixed;
            margin: 0;
            padding: 0;
            border: 0;
            outline: 0;
            left: 0;
            top: 0;
            width: 100%;
            height: 100%;
            pointer-events: none;
            z-index: 1
        }
    </style>
    <script>
        window.requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame || window
            .webkitRequestAnimationFrame || window.msRequestAnimationFrame;
        var starDensity = .216;
        var speedCoeff = .05;
        var width;
        var height;
        var starCount;
        var circleRadius;
        var circleCenter;
        var first = true;
        var giantColor = '180,184,240';
        var starColor = '226,225,142';
        var cometColor = '226,225,224';
        var canva = document.getElementById('universe');
        var stars = [];

        windowResizeHandler();
        window.addEventListener('resize', windowResizeHandler, false);

        createUniverse();

        function createUniverse() {
            universe = canva.getContext('2d');
            for (var i = 0; i < starCount; i++) {
                stars[i] = new Star();
                stars[i].reset();
            }
            draw();
        }

        function draw() {
            universe.clearRect(0, 0, width, height);

            var starsLength = stars.length;

            for (var i = 0; i < starsLength; i++) {
                var star = stars[i];
                star.move();
                star.fadeIn();
                star.fadeOut();
                star.draw();
            }

            window.requestAnimationFrame(draw);
        }

        function Star() {

            this.reset = function() {
                this.giant = getProbability(3);
                this.comet = this.giant || first ? false : getProbability(10);
                this.x = getRandInterval(0, width - 10);
                this.y = getRandInterval(0, height);
                this.r = getRandInterval(1.1, 2.6);
                this.dx = getRandInterval(speedCoeff, 6 * speedCoeff) + (this.comet + 1 - 1) * speedCoeff *
                    getRandInterval(50, 120) + speedCoeff * 2;
                this.dy = -getRandInterval(speedCoeff, 6 * speedCoeff) - (this.comet + 1 - 1) * speedCoeff *
                    getRandInterval(50, 120);
                this.fadingOut = null;
                this.fadingIn = true;
                this.opacity = 0;
                this.opacityTresh = getRandInterval(.2, 1 - (this.comet + 1 - 1) * .4);
                this.do = getRandInterval(0.0005, 0.002) + (this.comet + 1 - 1) * .001;
            };

            this.fadeIn = function() {
                if (this.fadingIn) {
                    this.fadingIn = this.opacity > this.opacityTresh ? false : true;
                    this.opacity += this.do;
                }
            };

            this.fadeOut = function() {
                if (this.fadingOut) {
                    this.fadingOut = this.opacity < 0 ? false : true;
                    this.opacity -= this.do / 2;
                    if (this.x > width || this.y < 0) {
                        this.fadingOut = false;
                        this.reset();
                    }
                }
            };

            this.draw = function() {
                universe.beginPath();

                if (this.giant) {
                    universe.fillStyle = 'rgba(' + giantColor + ',' + this.opacity + ')';
                    universe.arc(this.x, this.y, 2, 0, 2 * Math.PI, false);
                } else if (this.comet) {
                    universe.fillStyle = 'rgba(' + cometColor + ',' + this.opacity + ')';
                    universe.arc(this.x, this.y, 1.5, 0, 2 * Math.PI, false);

                    //comet tail
                    for (var i = 0; i < 30; i++) {
                        universe.fillStyle = 'rgba(' + cometColor + ',' + (this.opacity - (this.opacity / 20) * i) + ')';
                        universe.rect(this.x - this.dx / 4 * i, this.y - this.dy / 4 * i - 2, 2, 2);
                        universe.fill();
                    }
                } else {
                    universe.fillStyle = 'rgba(' + starColor + ',' + this.opacity + ')';
                    universe.rect(this.x, this.y, this.r, this.r);
                }

                universe.closePath();
                universe.fill();
            };

            this.move = function() {
                this.x += this.dx;
                this.y += this.dy;
                if (this.fadingOut === false) {
                    this.reset();
                }
                if (this.x > width - (width / 4) || this.y < 0) {
                    this.fadingOut = true;
                }
            };

            (function() {
                setTimeout(function() {
                    first = false;
                }, 50)
            })()
        }

        function getProbability(percents) {
            return ((Math.floor(Math.random() * 1000) + 1) < percents * 10);
        }

        function getRandInterval(min, max) {
            return (Math.random() * (max - min) + min);
        }

        function windowResizeHandler() {
            width = window.innerWidth;
            height = window.innerHeight;
            starCount = width * starDensity;
            circleRadius = (width > height ? height / 2 : width / 2);
            circleCenter = {
                x: width / 2,
                y: height / 2
            }

            canva.setAttribute('width', width);
            canva.setAttribute('height', height);
        }
    </script>
</html>
